1Department of Periodontology, College of Dentistry, Yonsei University, Korea.
2Reasearch Institute for Periodontal Regeneration, Korea.
3Brain Korea 21 project for Medical Science, Korea.
Corresponding author: Jung-Kiu Chai, Department of Periodontology, College of Dentistry, Yonsei University, Shinchon-dong, Seodaemun-gu, Seoul, 120-752, Korea.